We had the fire service come round ours and they installed the Fire Angel ST-750 [www.fireangel.co.uk
]. You can't get any easier to install than that.
Most of the battery operated ones are all pretty similar to install: find a joist in the ceiling in an appropriate place, screw the smoke alarm's base plate to it, attach smoke alarm and test.
How to Install A Fire Alarm [www.youtube.com